* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
.mask {
  width: 600px;
  height: 300px;
  background-color: lightblue;
  display: flex;
  justify-content: center;
  align-items: center;
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
}